1. Field
The patent specification relates to an X-ray detection device which is used in an X-ray CT (computerized tomography) apparatus, and, in particular, relates to a multi element solid state X-ray detection device with a high spatial resolution and a high S/N ratio which is suitable for concurrently detecting X-ray transmission data of a plurality of slices of an object body being inspected
2. Related Art
Currently, the X-ray detection device used for an X-ray CT apparatus tends to be a solid state detection device using a scintillator that has improved X-ray detection accuracy in comparison with conventional X-ray detection using a xenon ionization detector, Such a solid state detection device comprises a multiplicity of channels of X-ray detection elements arranged in an arcuate shape around an X-ray source, each element being formed by a scintillator for converting incident X-ray into light and a light detection element such as a silicon photo diode which detects the light converted by the scintillator and outputs the same as an electrical signal.
In an X-ray CT apparatus, in order to improve throughput of the device, it is desired to shorten the time required to obtain CT images. The following two methods are generally enumerated;
(1) Shorten the time required for a rotation of a scanner.
(2) Increase the number of tomographic images taken for every rotation of a scanner.
With regard to (1) above, an improvement in rotating speed of the scanner can be achieved by reducing the weight of an X-ray tube serving as an X-ray generating device. On the other hand, the above (2) can be achieved by arranging a row of X-ray detection elements for an X-ray detection device conventionally arranged in one dimension in the channel direction in a plurality of rows, in that two rows or more than two rows are arranged along a slice direction (which is perpendicular to the channel direction).
Such an X-ray detection device is called a multi slice type X-ray detection device (the conventional X-ray detection device in which the X-ray detection elements are simply arranged in one dimension is generally called a single slice type X-ray detection device).
FIG. 8 shows a schematic diagram of an example of such a multi slice type X-ray detection device applied to a CT apparatus. In FIG. 8, a relationship between a multi slice type X-ray detection device 13, a body 11 to be inspected and an X-ray tube 10 is illustrated. The multi slice type X-ray detection device 13 has four rows of X-ray detection elements 12, from element row 1 to element row 4 arranged in slice direction, and can measure image data for a region covering four slices from slice 1 to slice 4 of the body 11 by concurrently receiving X-ray beams 14 irradiated from an X-ray tube 10. As a result, the utilization efficiency of the X-ray beams 14 from the X-ray tube 10 is improved four times in comparison with the conventional single slice type X-ray detection device. Further improvements in efficiency can be achieved as the number of X-ray detection element rows 12 increases.
With the background of conventional single slice type X-ray detection device, many of such multi slice type X-ray detection devices are constituted by simply arranging several single slice type X-ray detection device in the slice direction. However, in such a multi slice type X-ray detection device, the respective X-ray detection elements must match in performance. If they do not match sufficiently well, ring artifacts can appear in the reconstructed CT images and thereby deteriorate image quality.
Further, when the performance of X-ray detection elements varies in the slice direction, the measured data can differ depending on which X-ray detection element row in the slice direction obtained the measurements; therefore, it is possible, even when the measurements of image data are performed with regard to the same slice plane of the body 11, that the image quality of the CT images and medical information obtained from the CT images would vary. The obtained CT images should not differ because of differences in performance of X-ray detection elements used for the measurement, when image data are measured with regard to a same slice plane of a same body 11. For this reason, it is required that the X-ray detection elements are sufficiently matched in performance in bot the channel an the slice directions.
For the reasons explained above, it can be difficult to manufacture a multi slice type X-ray detection device that performs well.
Further, in order to match the performance characteristic values of the respective X-ray detection elements, it is important to reduce electrical cross talk as well as optical cross talk between respective neighboring elements in the detection device in addition to matching the performance characteristics of the scintillators and silicon photo diodes included in the respective elements.
FIG. 4 is a perspective view showing a basic structure of a conventional single slice type X-ray detection device.
In FIG. 4, numeral 1 is a scintillator which converts incident X-ray 5 into light, numeral 2a is an isolation wall between neighboring X-ray detection elements and numeral 3 is a silicon photo diode array which converts the light converted by the scintillators 1 into electrical signals. Each of the X-ray detection element is constituted by adhering a scintillator 1 on the upper surface of a respective photo receiving portion provided on the surface of the silicon photo diode array 3, and an X- ray detection element array is constituted by arranging the thus constituted X-ray detection elements in parallel with a predetermined pitch on a circuit substrate 6 while interleaving the isolation walls 2a therebetween. Further, numeral 7 is an upper face reflection plate which efficiently reflects light from the scintillators 1 and introduces the same toward the respective photo receiving portions on the silicon photo diode array 3.
In the above structured X-ray detection device, the incident X-ray on and into the detection device is converted by the scintillators 1 into visible light having a local intensity proportional to the local intensity of the incident X-ray 5. The converted light is transmitted through the scintillators 1 in part through reflection such as at the surface of the upper face reflection plate 7, the surfaces of the isolation walls 2a and boundaries and surfaces of the scintillators 1, and is introduced onto the photo receiving portions provided on the surface of the silicon photo diode array 3 in which a photo-electric conversion is performed and electrical signals (photo currents) having intensities proportional to the intensity of light, namely proportional to the intensity of X-ray is detected.
The performance of an X-ray detection device is evaluated primarily depending on S/N ratio and spatial resolution thereof. The S/N ratio is determined by the contribution rate of the incident X-ray 5 on the output signal, namely by the X-ray utilization efficiency and electrical signals (noise signals) induced in an electrical circuit system including the silicon photo diode array 3 when no X-ray is incident into the X-ray detection device. Then, the X-ray utilization efficiency is determined by a luminous efficiency (light conversion efficiency) of the scintillators 1, a light conversion efficiency of the silicon photo diode array 3, a spatial utilization efficiency which represents a spatial X-ray utilization efficiency by the X-ray detection device and a light transmission efficiency in the X-ray detection device.
Noise signals are primarily caused by shot noise and dark currents due to recombination currents in depletion layers in the silicon photo diode array 3 and noise currents in the electric current system such as a preamplifier. Among the causes which affect the above X-ray utilization efficiency, the luminous efficiency (light conversion efficiency) of the scintillators 1 and the light conversion efficiency of the silicon photo diode array 3 are determined in one to one relationship based on their physical properties. The spatial utilization efficiency can be improved by reducing the size of a region which does not contribute for the detection of the incident X-ray 5, such as spaces occupied by the solation walls 2a isolating between the respective X-ray detection elements, namely, the regions other than the scintillators 1. The light transmission efficiency can be improved by taking in more light into the light receiving portions, such as by reducing self absorption of light within the scintillators 1 and absorption of light at the surfaces of such as upper face reflection plate 7 and the isolation walls 2a as well as by reducing light reflection at the respective surfaces by providing an efficient optical coupling between the surfaces of scintillators 1 having a large refractive index and the surfaces of the light receiving portions of the silicon photo diode array 3.
The parameters which control the spatial resolution of the images to be reconstructed by making use of the X-ray data obtained with such X-ray detections devices are, from geometrical point of view, a distance between neighboring isolation walls 2a as shown in FIG. 4, defining an opening width of the respective X-ray detection elements, and, from electrical and optical points of view, are leakage of electrical signals between neighboring X-ray detection elements (hereinbelow called electrical cross talk) and leakage of X-ray or light therebetween (herein below called optical cross talk). Further, an image noise due to deterioration of S/N ratio can also be considered.
At first, with regard to the opening width of the respective X-ray detection elements representing the geometrical parameter, if the opening width is narrowed, the spatial resolution can be improved, In response to demands for higher spatial resolution, recent opening widths for the X-ray detection elements are below 1 mm. However, there is a limitation in that the amount of incident X-rays into the respective X-ray detection elements decreases when the opening is narrowed, thereby, the level of output signals from the respective X-ray detection elements reduces and the S/N ratio also decreases.
Further, when the opening width is narrowed, a ratio of the space occupied by the isolation walls 2a and the X-ray detection elements is reduced, thereby, the spatial utilization efficiency is also reduced, Still further, when the opening width is narrowed, the attenuation amount of the light generated from the scintillators 1 increases because of the self absorption thereof within the scintillators 1 and of a high repetition rate of light reflection at the surface of the upper face reflection plate 7 and at the surfaces of the isolation walls 2a, and the light transmission decreases. As a result, the level of the output signals of the X-ray detection device decreases and accordingly the S/N ratio also decreases.
When the electrical cross talk and the optical cross talk between the neighboring X-ray detection elements increase, the spatial resolution thereof decreases, and in order to reduce such cross talk the following measures have been taken conventionally. In order to reduce electrical cross talk in a multi elements silicon photo diode array, a measure as illustrated in FIG. 5 has been taken.
FIG. 5 shows a cross sectional structure of a common PIN type multi element silicon photo diode array. In FIG. 5, numeral 3a is an N+ layer of n type semiconductor, numeral 3b is anxe2x80x94layer of an intrinsic semiconductor which is formed by reducing the impurity density in a region spreading of a depletion layer and numeral 3c shows P+ layers of p type semiconductor serving as light receiving regions and being provided at a plurality of regions on the surface of the silicon photo diode array 3. Between the adjacent light receiving regions 3c I(Nxe2x88x92) layer 3bxe2x80x2 of dead zone is provided so as to electrically isolate the adjacent elements. Further, in order to ensure the electrical isolation by the I(Nxe2x88x92) layer 3bxe2x80x2 of dead zone, local n type semiconductor regions 3axe2x80x2 and a local p type semiconductor region 3cxe2x80x2 are formed in the dead zone 3bxe2x80x2 for an isolation. Further, in FIG. 5 numeral 3d is an anode electrode for an individual photo diode element in that a silicon photo diode, numeral 3e is a reflection preventing film for the respective silicon photo diodes and numeral 3f is a protective oxide film.
Further, as measures for preventing optical cross talk between neighboring elements, a measure as illustrated in FIG. 6 has been proposed, where a light reflecting member 2b of white color pigment serving as an isolation wall is filled in a narrow gap between the neighboring scintillators 1 as disclosed in JP-A-58-219471(1983), and another measure as illustrated in FIG. 7 has been proposed, where a metal isolation wall 2bxe2x80x2 is inserted between the neighboring scintillators 1 as disclosed in JP-B-2720159(1997), and JP-B-2720162(1997).
The above referred to conventional measures for reducing the optical cross talk present the following problems.
In the structure as shown in FIG. 6 and as disclosed in JP-A-58-219471(1983), optical cross talks between scintillators 1 is reduced by means of reflecting members 2b between the scintillators 1. It is technically difficult to uniformly fill the light reflecting member 2b serving as an isolation wall in the narrow gaps between the scintillators 1 and when bubbles are mixed therein, which causes variation in sensitivity, nonuniform sensitivity distribution and optical leakage can occur. Further, since there is no optical isolation at an adhesion layer 4 adhering the scintillators 1 and the silicon photo diode array 3, a problem arises that an optical leakage is caused through the adhesive layer 4.
Further, in the structure as illustrated in FIG. 7 and as disclosed in JP-B-2720159(1997), after adhering the scintillators 1 onto the silicon photo diode array 3 via the adhesive layer 4, gaps for isolating the respective X-ray detection elements are formed up to the inside of the silicon photo diode array 3 so as to insert metallic isolation walls 2bxe2x80x2, therefore, a possible positional deviation between the respective scintillators 1 and the respective silicon photo diodes in the silicon photo diode array 3 is prevented, and in addition since the metallic isolation walls 2bxe2x80x2 are inserted inside the silicon photo diode array 3, an advantage of preventing optical leakage between neighboring X-ray detection elements is achieved. On the other hand, because of microcracks caused when the surface of the silicon photo diode array 3 is cut, leakage currents and dark currents in the silicon photo diodes increase which causes problems such as reducing S/N ratio and characteristic deviation of the respective X-ray detection elements. When applying the conventional measures for reducing electrical and optical cross talks having the above explained problems to a multi slice type X-ray detection device, the following problems will be further caused.
Namely, for constructing a multi slice type X-ray CT apparatus, it is necessary to form a two dimensional multi elements X-ray detection device in which X-ray detection elements are arranged in a plurality of rows in channel and slice directions as illustrated in FIG. 8. When realizing such two dimensional multi elements X-ray detection device, it is necessary to constitute the silicon photo diode array serving as a photo-electric conversion element in a two dimensional multi elements structure, and when it is required to improve the spatial resolution with such two dimensional multi elements structure, it is necessary to increase packing density of the silicon photo diode array.
Accordingly, in the multi slice type X-ray detection device, it is necessary to provide measures for increasing the packing density thereof such as decreasing an area occupied by the silicon photo diode array as well as using a part of the array as a region for wirings. However, If the conventional measures as disclosed such as in JP-A-58-219471(1983) and JP-B-2720159(1997) are used for the multi slice type X-ray detection device including above explained packing requirements, an improvement in packing density while ensuring a predetermined spatial resolution and S/N ratio will be limited.
An object of the present invention is to provide an X-ray detection device, in particular a multi element solid state X-ray detection device which is suitable for a multi slice type X-ray CT apparatus which reduces electrical and optical cross talk between respective neighboring X-ray detection elements, increases a packing density, equalizes the characteristics of the respective X-ray detection elements, and to provide an X-ray CT apparatus using such X-ray detection device.
The above object is achieved by one of or a combination of the following measures.
(1) In a multi element solid state detection device which is provided with an X-ray detection element array constituted by an array of photo diodes for multi channels arranged with a predetermined pitch on a substrate, a plurality of scintillators each being adhered onto the respective photo diodes for every channel and isolation walls disposed between the neighboring scintillators for respective channels, an isolation wall or band for isolating the respective channels is provided between respective light receiving portions of the photo diode array for the multi channels, and the surface of the isolation wall or band is covered by a material having light absorbing property.
(2) The width of region which is covered by the material having light absorbing property is equal to a region occupied by the width of the isolation wall provided between neighboring scintillators for the respective channels, smaller than a width region of respective scintillators including the corresponding isolation walls not contributing to X-ray detection, or larger than the thickness of the adhesive layer adhering the photo diode array and the scintillators for every channel.
(3) A reflection preventing film is provided on the surface of the respective light receiving portions of the photo diode array, and the refractive index of the adhesive layer adhering the photo diode array and the scintillators is smaller than the refractive index of the scintillators and the refractive index of the reflection preventing film.
(4) The isolation wall or band includes on the surface thereof a region covered by the material having light absorbing property and another region covered by a tight reflecting film.
(5) The width of the region on the surface of the isolation wall or band including the region covered by the material having light absorbing property and the region covered by the light reflecting film is larger than the width of the isolation wall.
(6) The material having light absorbing property provided on the surface of the isolation wall or band is carbon or carbon compound.
(7) The material having light absorbing property provided on the surface of the isolation wall or band is one selected from the group of sulfides such as Ag2S, FeS, NiS and Mo2S3 or any combinations thereof.
(8) The material having light absorbing property provided on the surface of the isolation wall or band is one selected from the group of oxides such as 0s0, CrO, SnO, Teo, Pb20, NbO, BiO, MoO and RuO or any combinations thereof.
(9) An X-ray CT apparatus which uses the X-ray detection device as referred to (1)xcx9c(8) above.
With the thus structured X-ray detection device according to the present invention, light generated in the respective isolated scintillators that would leak through the adhesive layer to the respective neighboring scintillators or the respective neighboring light receiving portions on the photo diode array, is absorbed by the material having light absorbing property provided on the surface of the isolation wall or band. Accordingly, the light which reaches the isolation wall or bands is substantially absorbed by the light absorbing material, and light leakage between the neighboring X-ray detection elements is eliminated, thereby, the spatial resolution of the device is enhanced. Further, it is unnecessary to cut and separate the respective X-ray detection elements down to the inside of the silicon photo diode array as has been explained in connection with JP-B-2720159(1997), and so deterioration in S/N ratio and deviation of characteristics of the respective X-ray detection elements induced by increased leakage currents and dark currents in the respective silicon photo diodes due to causes such as microcracks caused during the processing thereof are also eliminated. Accordingly, an X-ray detection device which permits an efficient conversion of X-ray energy into electrical signals can be realized, thereby, when such an X-ray detection device is applied to an X-ray CT apparatus, in particular to a multi slice type X-ray CT apparatus which necessitates great many number of X-ray detection elements, tomographic images of high quality for a plurality of slices can be obtained concurrently.